The present invention relates to an electroless gold plating solution and a method for electroless plating.
There have been developed a number of electroless gold plating solutions which can be used in a neutral range at lower temperatures, so that it can be applied for a wider range of resists and electronic parts to be plated, as a substitute for a conventional electroless gold plating solution which is strongly alkaline and used at a high temperature. These plating solutions disadvantageously have a low stability and a poor deposition property. The causes for lowering stability of a plating solution are roughly classified into two types, namely, lowering of stability of the electroless gold plating itself and lowering of stability due to impurity metals mixed into the plating solution during a plating operation. For solving these problems, a number of attempts have been made to improve them.
In order to carry out an electroless gold plating in a neutral range without using a cyan compound, Japanese Prov. Patent Publication No. 191782/1989 discloses use of ascorbic acid as a reducing agent in the plating solution.
In addition, for suppressing mixing of impurity metals into the plating solution during a plating operation, or for improving stability of the plating solution, Japanese Prov. Patent Publication Nos. 350172/1992 and 145997/1994 disclose addition of a mercaptobenzothiazole based compound as a metal-shielding agent to the plating solution.
Further, Japanese Prov. Patent Publication No. 215677/1991 discloses use of a hydrazine compound (10 to 30 g/l) as a reducing agent in an electroless gold plating solution, and this plating bath can achieve a practical deposition rate in a lower concentration of the reducing agent, as compared to the above plating bath using ascorbic acid.
Furthermore, Japanese Prov. Patent Publication No. 314871/1992 discloses that the electroless gold plating solution is improved by addition of a benzotriazole based compound as a metal-shielding agent which is used for suppressing mixing of impurity metals into the plating solution during a plating operation or for improving stability of the plating solution, and that this shielding agent can be practically used in a wide use range (3 to 10 g/l).
On the other hand, Japanese Patent No. 2972209 discloses use of a thiourea compound or a phenyl compound as a reducing agent in the plating solution, and that thiourea can reduce gold in a low concentration. However, the thiourea has a problem that by-products of thiourea deteriorate stability of the plating solution and decompose the plating solution. In addition, the phenyl compound based reducing agent has a problem that it cannot reduce gold in a neutral range (pH 7 to 7.5) and hence it is inevitably used in a weakly alkaline range, so that the plating solution is decomposed during the plating. For solving the problems, Japanese Prov. Patent Publication No. 104877/1991 proposes an electroless gold plating solution containing both the thiourea compound and the phenyl compound as reducing agents, and this plating solution is improved in stability since the phenyl compound based reducing agent reduces a by-product of thiourea.
Further, Japanese Prov. Patent Publication No. 157859/1997 discloses that the above electroless gold plating bath is improved by addition of a benzotriazole based compound as a metal-shielding agent which is used for suppressing mixing of impurity metals into the plating solution during a plating operation or for improving stability of the plating solution, and that this plating bath is improved in stability, as compared to a conventional plating bath.
Ascorbic acid has a low reducing efficiency as a reducing agent. Therefore, ascorbic acid has a problem that it must be used in a sodium ascorbate concentration as high as 60 to 100 g/l for securing a practical deposition rate, i.e., 0.5 to 1.0 xcexcm, thus lowering stability of the plating solution.
The mercaptobenzothiazole compound as a metal-shielding agent has a problem that it has a very narrow use range (0.1 to 5 ppm) and hence exhibits low operation efficiency, and deposition failure arises when it is used in a larger amount.
When a hydrazine compound is used as a reducing agent, the resultant plating bath can achieve a practical deposition rate in a low reducing agent concentration, as compared to the plating bath using ascorbic acid. However, the hydrazine compound has itself a poor stability, failing to secure a sufficient stability of the plating solution. This plating bath is improved by addition of a benzotriazole based compound as a metal-shielding agent which is used for suppressing mixing of impurity metals into the plating solution during a plating operation or for improving stability of the plating solution. However, the stability of the reducing agent itself is poor as mentioned above, and as a result, stability of the plating solution cannot be improved enough for a practical use.
The electroless gold plating solution containing both the thiourea compound and the phenyl compound as reducing agents is improved in stability by reducing by-products of thiourea with the phenyl compound based reducing agent. However, it has a problem that the by-products of thiourea cannot be completely reduced to the original reducing agent and the remaining by-products causes deposition failure or lowered stability of the plating solution, thus making it difficult to secure a sufficient stability of the plating solution.
It is an object of the present invention to provide an electroless gold plating solution and a method for electroless gold plating, which can maintain a practical deposition rate with a reduced amount of a reducing agent and achieve excellent stability of the plating solution.
In order to attain the above-mentioned objects, the present inventors have selected a phenyl compound based reducing agent having a high reducing efficiency, so that stability of a plating solution is less spoiled by by-products produced in reduction process, and they have made intensive and extensive studies. As a result, they have found that the presence of a water-soluble amine such as ethylene diamine, etc. is surprisingly improve deposition rate of a neutral electroless gold plating solution (pH 7.0 to 7.5) using the phenyl compound based reducing agent as a reducing agent, whereby enabling an electroless gold plating at a rate of around 1 xcexcm/h, and provide an electroless gold plating solution having an excellent plating appearance and an excellent solution stability, without spoiling appearance and deposition property.
The present invention is characterized as follows.
(1) A gold plating solution comprising a gold salt, a phenyl compound based reducing agent and a water-soluble amine.
(2) The electroless gold plating solution according to (1), wherein the phenyl compound based reducing agent is represented by the formula (I): 
wherein R1 represents a hydroxyl group or an amino group, R2 to R4 may be the same or different, and independently represent a hydroxyl group, an amino group a hydrogen atom or an alkyl group.
(3) The electroless gold plating solution according to (2), wherein the alkyl group for R2 to R4 is methyl group, ethyl group, or t-butyl group.
(4) The electroless gold plating solution according to (1) or (2), wherein the phenyl compound based reducing agent is hydroquinone, methylhydroquinone, or p-phenylenediamine.
(5) The electroless gold plating solution according to any one of (1) to (4), wherein the water-soluble amine is ethylene diamine based compound.
(6) The electroless gold plating solution according to any one of (1) to (5), which further comprises an impurity metal-shielding agent as an additive.
(7) The electroless gold plating solution according to (6), wherein the impurity metal-shielding agent is benzotriazole based compound.
(8) The electroless gold plating solution according to any one of (1) to (7), wherein a pH of the electroless gold plating solution is in the range of 5 to 10.
(9) A method for electroless plating, comprising immersing a material to be plated into a gold plating solution which comprises a gold salt, a phenyl compound based reducing agent and a water-soluble amine.